The principle of Soxhlet extraction is not new.
Invented by Franz von Soxhlet, a German agricultural chemist who died in 1926, Soxhlet apparatus has long been used in extracting fatty or other material with a volatile solvent (e.g. ethyl ether, alcohol or benzene).
A Soxhlet apparatus comprises a vertical glass cylindrical extraction tube that has both a siphon tube and a vapor tube, that is fitted at its upper end to a reflux condenser and at its lower end to a flask so that the solvent may be distilled from the flask into the condenser. From the condenser the solvent in liquid phase flows back into the cylindrical tube which holds the sample to be extracted in a porous thimble. When the solvent level rises to the top of the siphon tube the solvent, with the extracted materials, is siphoned over into the flask, to be distilled again, and thus start another cycle.
Other solvents, such as carbon disulfide, pentane, and Freon have also been used in Soxhlet extraction.
After a sufficient period of time, all extractable matter in the sample is located in the flask, along with a major portion of the solvent, which is usually removed by evaporation. Depending on the amount of solvent and its boiling point, evaporation as a process for removing the solvent can result in appreciable losses of volatile constituents of the sample.
Fixed gases, such as carbon dioxide, can also be obtained as liquids at a suitably elevated pressure. Carbon dioxide, being molecularly similar to carbon disulfide, is also an excellent extractant. Owing to its low boiling point (-80.degree. C.) it can be removed at low temperature without the loss of volatile sample constituents.
Carbon dioxide has in fact been previously utilized as an extractant (see, for example, Schultz et al, Pilot-Plant Extraction With Liquid CO.sub.2, 32 Food Technology, June 1974) but in most instances has required the fabrication of highly specialized and expensive apparatus.
There is, in other words, considerable room for improvement.